In combustion chambers of turbine engines, pressure or acoustic vibrations can occur during the combustion process under certain conditions. The vibrations may range in frequencies from about twenty hertz to a few thousand hertz, and may occur due to instabilities in the combustion process. The lower frequency acoustic vibrations are sometimes referred to as “rumble” or “chugging.” Acoustic vibrations having frequencies higher than about 1000 hertz are typically referred to as “screech.” Screech has been found to interfere with optimal operation of the turbine engine. Once screech occurs, it can continue until the source of energy causing the screech is removed, or until system variables are changed, to shift the operation of the turbine engine to a non-screech operational range. However, changing the operational characteristics of the turbine engine to eliminate screech may be difficult. Since the mechanics of how the operational characteristics interact to produce screech is only minimally understood, it is extremely difficult to predict screech in a system with sufficient accuracy. Therefore, a positive structural means is often designed into the combustion chamber to damp the high frequency vibrations or cancel them out completely. One structural element which may be included in the combustion chambers to reduce screech of turbine engines is called a Helmholtz resonator.
A Helmholtz resonator is based on a device created by Hermann von Helmholtz in the 1860s, and works on the phenomenon of air resonance within a cavity. A Helmholtz resonator, in its simplest form, consists of an enclosed volume (cavity) containing air connected to the combustion chamber with an opening. Due to a pressure wave resulting from the combustion process, air is forced into the cavity increasing the pressure within. Once the external driver that forced the air into the cavity is gone, the higher pressure in the cavity will push a small volume of air (plug of air) near the opening back into the combustion chamber to equalize the pressure. However, the inertia of the moving plug of air will force the plug into the combustion chamber by a small additional distance (beyond that needed to equalize the pressure), thereby rarifying the air inside the cavity. The low pressure within the cavity will now suck the plug of air back into the cavity, thereby increasing the pressure within the cavity again. Thus, the plug of air vibrates like a mass on a spring due to the springiness of the air inside the cavity. The magnitude of this vibrating plug of air progressively decreases due to damping and frictional losses. The energy of the pressure wave generated within the combustion chamber is thus dissipated by resonance within the Helmholtz resonator. Energy dissipation is optimized by matching the resonance frequency of the resonator to the acoustic mode, of the combustion chamber enclosure, that is being excited. Typically, frequency matching, or “tuning,” of a Helmholtz resonator is accomplished by changing the dimensions of the Helmholtz cavity and opening.
An array of Helmholtz resonators is usually constructed using an empty space between interior and exterior liners of a double dome combustion chamber (combustor). At this location, the Helmholtz resonators are close to a heat release zone of the combustion chamber that creates the instabilities and are, therefore, suitably positioned to quickly respond to the resulting acoustic waves. However, in most combustion chambers, the space between the liners is also used to supply cooling air to the combustion chamber walls, and placing the Helmholtz resonators in this space makes them a part of a cooling system. Helmholtz resonators being a part of the cooling system, however, reduces the ability to tune the Helmholtz resonators by changing the cavity and opening dimensions, without impacting cooling of the combustion chamber. This limitation reduces the effectiveness of the Helmholtz resonators in controlling screech. It is therefore desirable to locate the Helmholtz resonators close to the heat release zone, but independent of the combustion chamber cooling system.
One implementation of a Helmholtz resonator in a gas turbine combustion chamber is described in U.S. Pat. No. 5,431,018 (the '018 patent) issued to Keller on Jul. 11, 1995. The Helmholtz resonator of the '018 patent is disposed around an air shroud that feeds the air necessary for mixing with fuel. Part of the air from the air shroud is bypassed into the Helmholtz resonator using an inlet tube. The Helmholtz resonator is connected to a combustion chamber using a damping tube that is configured as an annular duct around the air shroud. The '018 patent, thus, discloses a single Helmholtz resonator that is formed by a cavity around each fuel injector and connected to the combustion chamber by an annular opening around the injector while being independent of a combustion chamber cooling system of the combustion chamber.
Although the Helmholtz resonator of the '018 patent may be disassociated from the combustion chamber cooling system, it may be associated with the fuel injector air flow. Therefore, varying air flow through the fuel injector in response to changing output requirements of the turbine engine may affect the effectiveness of this resonator. In addition, tuning the resonator of the '018 patent to match the natural frequency of the turbine engine may involve redesigning the annular duct and/or the fuel injector. Typically, tuning the Helmholtz resonator to the appropriate frequency is a trial-and-error process that may involve experiments using a number of configurations (cavity volume, size of the opening that connects the cavity to the combustion chamber, etc.) of the resonator. Thus, it may be advantageous to have the ability to easily test different resonator configurations during development of the system.
The present disclosure is directed at overcoming one or more of the shortcomings set forth above.